Coordinated multi-point (CoMP) transmission is a very promising technique for an LTE-Advanced communication system to effectively reduce inter-cell interference (ICI) and improve the coverage of high data rates and cell-edge throughput and/or increase system throughput. Multi-site joint transmission (JT) is one of CoMP schemes, where data transmission intended for one or more UEs is shared within a CoMP cooperating set and is jointly processed among multiple cells. Channel information of all the UEs is acquired and shared through channel quantization feedback and backhaul exchange. Therefore, joint multi-site proportional fair (PF) scheduling and centralized zero-forcing (ZF) precoding can be performed among the coordinated eNBs or in a centralized scheduler. Multiple UEs can be jointly served by multiple eNBs in the CoMP cooperating set through coherent transmission so as to improve signal power and reduce inter-cell interference (ICI). It is obviously that channel information feedback is an important factor in the multi-site joint transmission scheme.
Therefore, there is a need for an efficient channel feedback mechanism, which can not only fully support multi-site joint transmission, but also have satisfactory backward compatibility with single cell SU/MU-MIMO (single user/multi-user multiple-input and multiple-output) or sub-optimal multi-site coordination schemes such as coordinated scheduling/coordinated beamforming etc.
The multi-site joint transmission scheme highly depends on channel information feedback from each UE. Each UE can report only one component channel information from multiple coordinated eNBs. In this way, a larger codebook size with a more transmit antenna number should be designed to better quantize the component channel information. On the other hand, if only the component channel information is fed back, the eNB cannot perform such schemes as single-cell SU/MU-MIMO scheduling or coordinated scheduling/beamforming etc. any more. Therefore a more efficient feedback mechanism should be explored during the design of the multi-site joint transmission scheme.